Impurity effects on adhesive energies

Some simple arguments are made about the effect of impurities on adhesive interactions at solid junctions. A universal adhesive force relation is derived for brittle adhesion. The adhesive binding energy, ΔE, is an important parameter in brittle adhesion forces. ΔE has also been shown by others to be important when there is plastic flow. We found that impurity effects on ΔE are determined by the segregation energies to the junction and to the free surfaces. At low temperatures, if it is energetically more favorable for impurities to segregate to the surfaces than to the junction, then the impurities will decrease ΔE. The converse is also true. For example, for self junctions which are in registry, ΔE is decreased if surface segregation is exothermic and increased if it is endothermic. These segregation energy relationships are consistent with the results of a number of experiments on the effects of impurities on adhesion forces and grain boundary embrittlement.     

Using Auger electron spectroscopy to assess the effect of segregation on the interfacial characteristics of polycrystalline copper

The possibility of using AES data on the composition of a free surface to assess surface and grain boundary tensions, and the grain boundary strength of polycrystalline copper, is discussed. Using familiar theoretical models, it is shown that sulfur impurity segregation substantially reduces these characteristics, with the energy of grain boundary fracture lowered by 5–8%.     

Kinetics of impurity segregation at grain boundaries in polycrystals. II. Concentrated solution

The isothermal impurity segregation from a finite-size grain into an intergrain boundary region or at an external free surface is investigated when the impurity concentration in the boundary region is not small. Simple algebraic equations are obtained that describe the impurity concentration for the case of several competing and interacting impurities. The process of segregation of two impurities is discussed in detail, and it is shown that the concentration of one of them can have a maximum as a function of time. Fiz. Tverd. Tela (St. Petersburg) 40, 251–253 (February 1998)     

Solute segregation to grain boundaries and free surfaces in an Fe---Si multicomponent alloy

Solute segregation was measured at both the {310} symmetrical tilt grain boundary and the (310) free surface of a sample of an Fe-6at%Si alloy containing traces of P, S, N and C at 873 K. Large phosphorus enrichment and silicon depletion characterize the grain boundary segregation in spite of a different bulk concentration of nitrogen. The surface segregation in nitrogen-containing samples is controlled by strong cosegregation of Si and N, resulting in the formation of a stable Si_xN_y 2D surface compound, whereas pronounced surface segregation of sulphur dominates in denitridized samples. The differences of grain boundary and surface segregation are discussed as a kind of “anisotropy of interfacial segregation” on the basis of Guttmann's theory with different values of free energies of segregation to grain boundary and free surface. They also suggest that the measurements of surface segregation cannot be unambiguously used for predicting the grain boundary segregation. In some non-brittle multicomponent systems, a better way of predicting segregation behavior at grain boundaries would be the measurement of grain boundary segregation in a related system with solute concentrations that cause embrittlement. The findings can then be applied to the required alloy composition on the basis of Guttmann's theory.     

An atomistic study of grain boundary segregation and cracking

A Monte Carlo model is applied to study a Σ = 5 (310) fcc tilt boundary structure and impurity segregation to this boundary. The Johnson-type potential functions are used to express atomic interactions for two binary alloys of NiCu and CuSb systems. Through atomic relaxation near absolute zero, the basic structural unit of the Σ = 5 CSL boundary is found to dissociate into two subunits. For the NiCu system, copper segregation to the boundary follows the Gibbsian equilibrium segregation behavior in which the segregation is localized on a few atomic layers, its amount increases with increase in the bulk concentration but decreases with increase in temperature. For the CuSb system, an assumption of the CuCu bond weakening due to the strong, adjacent CuSb bonds is also incorporated to realize the embrittling effect of Sb atoms. The preferential site for the large Sb atoms is the vertex of a pentagonal bipyramid in the Σ = 5 grain boundary. Without bond weakening, the Sb-segregated grain boundary maintains a structure combined with a distorted pentagonal bipyramid and a capped trigonal prism. However, with bond weakening some bonds of the trigonal prisms are disrupted in order to form new pentagonal bipyramids. When a uniaxial strain is applied in the direction perpendicular to the grain boundary plane, the weakened copper bonds become the source of microcracks thus enhancing brittle fracture along the Sb-segregated grain boundary.     

Electronic and Shallow Impurity States in Semiconductor Heterostructures Under an Applied Electric Field

With the use of variational method to solve the effective mass equation, we have studied the electronic and shallow impurity states in semiconductor heterostructures under an applied electric field. The electron energy levels are calculated exactly and the impurity binding energies are calculated with the variational approach. It is found that the behaviors of electronic and shallow impurity states in heterostructures under an applied electric field are analogous to that of quantum wells. Our results show that with the increasing strength of electric field, the electron confinement energies increase, and the impurity binding energy increases also when the impurity is on the surface, while the impurity binding energy increases at first, to a peak value, then decreases to a value which is related to the impurity position when the impurity is away from the surface. In the absence of electric field, the result tends to the Levine＇s ground state energy （-1/4 effective Rydberg） when the impurity is on the surface, and the ground impurity binding energy tends to that in the bulk when the impurity is far away from the surface. The dependence of the impurity binding energy on the impurity position for different electric field is also discussed.     

W-In体系溶质晶界偏聚行为的第一性原理计算

基于第一性原理构建了钨基合金体系的溶质偏聚模型,以W-In体系为例研究了不同浓度下溶质的晶界偏聚行为和成键特征,从电子结构层面揭示了W-In体系的键合作用,预测了W-In体系界面稳定性随溶质浓度的变化规律.结合键布居、电荷密度、差分电荷密度和态密度等电子结构分析,发现了W-In体系中溶质原子在偏聚过程中的键性转变特征,阐明了W-In键由晶粒内部的离子键过渡为晶界区域强共价键的微观机理.模型计算首次得到了W-In体系中溶质本征偏聚能随In浓度的非单调变化规律,结合键合作用和能量分析揭示了溶质浓度对本征偏聚能的影响机制.计算预测了W-In体系达到高热稳定性所需的最佳溶质浓度范围和应避开的溶质浓度范围.本研究为具有高温稳定性的钨基合金材料的设计与制备提供了理论基础和定量化指导.     

杂质离子对等离子体边界参量的影响

采用一维流体模型研究了含有杂质离子的等离子体与器壁材料相互作用给边界等离子体参量带来的影响.通过数值模拟,研究了分别选用碳和钨作为器壁材料时,器壁温度不同情形下热发射产生的电子对等离子体器壁电势、电场强度、热发射电子流以及沉积器壁离子动能流的影响.研究结果发现,当面向等离子体材料表面温度升高时,器壁电势和热发射产生的电流将增加,器壁电场强度和离子沉积器壁动能流则会减小,并且钨作为器壁材料要比碳作为器壁材料对于等离子体边界参量影响更明显.此外,研究了钨作为器壁材料时,碳杂质离子（浓度和电荷数）对等离子体器壁参量的影响.     

Anomalous dispersed states of alloys caused by segregation of impurities at phase interfaces

The diffusion in alloys with tendency to impurity segregation at phase interfaces has been simulated by the Monte Carlo method. It has been shown that at the segregation energy higher than the critical value, dispersity of the system increases due to the suppression of coagulation of phase particles. The effect mathematically represents the violation of the Lifshitz-Slyozov kinetics of an infinite growth of phase particles, which is similar to the anomalous kinetics of grain growth in systems with an impurity. The existence of the equilibrium grain size due to the equilibrium grain size caused by the impurity segregation at grain boundaries was previously predicted by Weissmuller and is now confirmed experimentally. Thus, this study generalizes the Weissmuller effect to the thermodynamics of the decomposition in alloys.     

Impurity effects on ionic-liquid-based supercapacitors

Small amounts of an impurity may affect the key properties of an ionic liquid and such effects can be dramatically amplified when the electrolyte is under confinement. Here the classical density functional theory is employed to investigate the impurity effects on the microscopic structure and the performance of ionic-liquid-based electrical double-layer capacitors, also known as supercapacitors. Using a primitive model for ionic species, we study the effects of an impurity on the double layer structure and the integral capacitance of a room temperature ionic liquid in model electrode pores and find that an impurity strongly binding to the surface of a porous electrode can significantly alter the electric double layer structure and dampen the oscillatory dependence of the capacitance with the pore size of the electrode. Meanwhile, a strong affinity of the impurity with the ionic species affects the dependence of the integral capacitance on the pore size. Up to 30% increase in the integral capacitance can be achieved even at a very low impurity bulk concentration. By comparing with an ionic liquid mixture containing modified ionic species, we find that the cooperative effect of the bounded impurities is mainly responsible for the significant enhancement of the supercapacitor performance.     

Theoretical aspects of solute segregation to high-angle grain boundaries

A high-angle grain boundary is modeled as a planar defect characterized by its thickness and atomic density. We successively examine the elastic and electronic contributions to the solute/grain boundary binding energy. We deduce the effect of the grain boundary physical parameters on its propensity for segregation. The thickness of high-angle grain boundaries is not a fundamental parameter for segregation. The atomic density in the grain boundary controls the electronic binding energy. The rate of change of elastic constants with the density is the important factor in the elastic contribution to segregation. We conclude that segregation to boundaries with small excess volumes is not precluded.     

Energy partition and segregation for an intruder in a vibrated granular system under gravity

The difference of temperatures between an impurity and the surrounding gas in an open vibrated granular system is studied. It is shown that, in spite of the high inhomogeneity of the state, the temperature ratio remains constant in the bulk of the system. The lack of energy equipartition is associated to the change of sign of the pressure diffusion coefficient for the impurity at certain values of the parameters of the system, leading to a segregation criterium. The theoretical predictions are consistent with previous experimental results, and also in agreement with molecular dynamics simulation results reported in this Letter.     

Interfacial segregation in Ag-Au,Au-Pd,and Cu-Ni alloys: II. [001] Σ5 twist grain boundaries

Atomistic simulations of segregation to [001] 5 twist boundaries in Cu–Ni, Au–Pd, and Ag–Au alloy systems have been performed for a wide range of temperatures and compositions within the solid solution region of these alloy phase diagrams. In addition to the grain boundary segregation profiles, grain boundary free energies, enthalpies, and entropies were determined. These simulations were performed within the framework of the free energy simulation method, in which an approximate free energy functional is minimized with respect to atomic coordinates and atomic site occupation. For all alloy bulk compositions (0.05 C 0.95) and temperatures (400 T (K) 1,100) examined, Cu and Au segregates to the boundary in the Cu–Ni and Au–Pd alloy systems, respectively; although in the Ag–Au alloys, the majority element segregates to the boundary. The width of the segregation profile is limited to approximately three to four (002) atomic planes. The classical theories for the segregation, and the effects of the relaxation with respect to either the atomic positions or the atomic concentrations, are discussed. The boundary thermodynamic properties depend sensitively on the magnitude of the boundary segregation, and some of them are shown to vary linearly with the magnitude of the grain boundary segregation.     

Proton-induced grain boundary segregation in stainless steel

Abstract

A technique is developed which addresses the problem of irradiation assisted stress corrosion cracking of stainless steels in light water reactors using high energy protons to induce grain boundary segregation. These results represent the first grain boundary segregation measurements in bulk produced by proton irradiation of stainless steel. The technique allows the study of grain boundary composition with negligible sample activation, short irradiation time, rapid sample turnaround and at minimal cost. Scanning Auger electron microscopy is used to obtain grain boundary composition measurements of irradiated and unirradiated samples of ultra high purity (UHP) type 304L stainless steel and UHP type 304L steels with the additions of phosphorus (UHP + P) and sulphur (UHP + S). Results show that irradiation of all three alloys causes significant Ni segregation to the grain boundary and Cr and Fe away from it. Irradiation of the UHP + P alloy also results in segregation of P at the grain boundary from 5.3 to 8.7 at %, over 80 times the bulk value. No radiation-induced grain boundary segregation of S was measured in the UHP + S alloy. Results also indicate that the presence of P or S may enhance radiation-induced segregation of major alloying elements at the boundary. Comparison of irradiated and unirradiated regions of the UHP + P alloy indicate that while a prior thermal treatment segregates P to the grain boundary to 5.3 at %, the major element concentrations at the grain boundary are completely different from those under irradiation.     

Measuring grain boundary segregation using Wavelength Dispersive X-ray Spectroscopy: Further developments

A technique for the quantification of surface and grain boundary segregation using Wavelength Dispersive X-ray Spectroscopy (WDS) in the Scanning Electron Microscope (SEM) is proposed. As an example, the case of sulphur segregation in nickel is considered. The sulphur segregation can be quantified using a single voltage measurement (20 kV) of the sulphur Kα line intensity. The quantification is made from a simple proportionality equation derived from the XPP microanalysis model of correction. The result of segregation quantification by WDS is a surface concentration expressed in g.cm^-2.Special attention was paid to the quantification of grain boundary segregation on fracture surfaces, taking into account the off-normal incidence of the electron beam on the analyzed surface. A simple technique that allows determination of the tilt angle from the specimen absorbed current is proposed. The influence of the azimuth angle of the analyzed surface with respect to WDS spectrometer is also discussed.The results of the WDS technique are shown to be repeatable within 5% with reasonable counting times (a few minutes per facet). As an example, the kinetics of sulphur grain boundary segregation in nickel at 750 °C was measured using WDS to document the quantitative capabilities of the technique.As already pointed out in a previous paper, it is confirmed that WDS is insensitive to surface contamination, which allows the analysis of ex-situ fractured samples.     

纤锌矿GaN柱形量子点中类氢施主杂质态

在有效质量近似和变分原理的基础上,选取含两个变分参数的波函数,研究了纤锌矿结构的GaN/AlxGa1-xN单量子点中类氢施主杂质体系的结合能随量子点(QD)尺寸以及杂质在量子点中位置的变化,并与以前使用不同尝试波函数的计算结果进行了比较。结果表明:由我们选取的两变分参数波函数得到的结果与前人选取的两变分参数波函数得到的结果相比有所改进,而与选取一个变分参数波函数得到的结果一致。同时我们还计算了体系的维里定理值随量子点半径的变化情况,所得结果与前人工作结果一致,说明本文选取的两变分参数波函数能很好地描述柱形量子点中施主杂质态的运动。     

Modelling of grain boundary impurity segregation during high temperature plastic deformation

A modified theoretical model is proposed to predict the grain boundary segregation of impurity atoms during high temperature plastic deformation. The model is based on the supersaturated vacancy-impurity complex created by plastic deformation and involves quasi-thermodynamics and kinetics. Model predictions are made for phosphorus grain boundary segregation during plastic deformation in ferrite steel. The results reveal that phosphorus segregates at grain boundaries during plastic deformation. At a given te...     

Computer simulation of solute segregation to a Σ = 5 tilt boundary in Au,Cu and Ni

A Lattice Energy Function that combines a Mie type interatomic potential and a free electron gas volume dependence has been applied to the study of grain boundary energy and structure of a Σ = 5 tilt boundary in Au, Cu and Ni and of solute segregation to the same. Interatomic potentials and volume dependencies of the solvent and solute were adjusted to fit the relative partial molar enthalpy and volume at infinite dilution order to construct a AB type potential and volume dependence. This AB interaction is then applied to calculate the binding energies of solute to various grain boundary sites and the resulting change in grain boundary energy. A relationship between the binding energy and change in grain boundary is derived. The relative values of the grain boundary energy are in agreement with experimental values of the average grain boundary energies. The relative binding energies of the tested solvent-solute systems are in agreemnet with expectations that certain systems should have larger binding energies than others. The behavior of solute binding energies and local relaxations are in agreement with other studies of grain boundary segregation which use different Lattice Energy Functions and relaxation algorithms. The change in grain boundary energy is shown to be directly proportional to the binding energy.     

Polaronic Effects on the Impurity Binding Energy in a Polar Crystal Slab Within an Electric Field

The effects of the interaction between electron and bulk longitudinal optical (LO) phonon and surface optical (SO) phonon on the impurity binding energy of the ground state in a polar crystal slab within an external electric field are derived by using the method of a variational wavefunction. The binding energy of the bound polaron is obtained as a function of the impurity position, the slab thickness and the electric field strength. It is found that the polaronic correction to the impurity binding energy by the SO phonon may be enhanced and that by the LO phonon may be reduced with increasing electric field strength. And the effect of the electron-phonon interaction is quite important in increasing the values of binding energy.